The next generation of PassivHaus design

Beautiful and low-carbon: three very different projects showcase the current state of PassivHaus design

First, a bit of myth-busting about the PassivHaus approach to low-carbon building. A PassivHaus project can be any building type, not just a house. It applies equally to new build and refurbishment. And – to dispel the most frequently cited misinformation – while it’s true that some advocates of PassivHaus obsess over triple-glazing and airtightness tests, windows can be opened in a PassivHaus project.

Buildings are designed with mechanical heat recovery ventilation (MHRV) so that it is not necessary to open them, but opening windows in summer and during shoulder seasons poses no problems, and the only downside to open windows in winter is the cold.

The availability of technical information, training and suitable products relating to PassivHaus has increased significantly in the UK since the AJ first reviewed the low-carbon building certification two years ago. Nearly 40 projects of all building types, including refurbishments, are complete or in the pipeline across the UK (see map). The launch of the PassivHaus Trust at a recent London conference devoted to the subject now provides a focus for technical guidance.

PassivHaus principles are simple: high levels of insulation detailed to minimise thermal bridging; careful exploitation of solar gain; a high level of airtightness and indoor air quality provided by MHRV. To be certified as PassivHaus in Europe between 40° and 60° latitude, a building’s total annual energy demand for heating and cooling must be less than 15kWh/m2.

The rigorous tests that accompany PassivHaus certification can seem restrictive to architects. Until recently a number of awkwardly-proportioned PassivHaus buildings with large expanses of south-facing glazing gave the certification a bad name. The projects on the following pages aim to bust that myth, too.

Camden PassivHaus, London, by Bere Architects

The project is a 120m2 single family house split over two floors, with two wild flower meadow roofs and a south-facing garden. The house superstructure was prefabricated in Austria. Walls are 240-280mm thick prefabricated insulated timber framed panels clad in the factory with European larch fixed to untreated softwood battens.

CO2 emissions have been minimised with insulation, draught-free construction, triple-glazed windows and a solar thermal panel. All details were thermally modelled by the architects to prevent cold bridges and eliminate cold spots that would risk mould growth and damp patches. Automatic solar shading by external venetian blinds on the south elevation will control heat gains in summer.

The house uses non-toxic materials throughout, together with heat-recovery ventilation and water filtration. Careful attention has been given to unregulated energy consumption; while any heat required will come from the air supply, a towel radiator has been installed in each ensuite bathroom. Towel rails are controlled by a switch with a 30, 60 or 120 minute option before switching off. An internal ventilated clothes-drying room and external concealed south-facing clothes drying space discourage tumble drying.

Tigh-Na-Cladach, Dunoon Argyle and Bute, by Gokay Deveci

Tigh-Na-Cladach means ‘house by the shore’. One of these 10 semi-detached units of affordable housing for Fyne Initiatives overlook the River Clyde has been PassivHaus approved. Achieving PassivHaus standard was particularly challenging because the house’s tall, slim shape creates an unfavourable volume-to-area ratio, has no south-facing windows (except for roof lights), no mains gas serves the site and the social housing budget was very tight.

All of the houses, which measure 88.4m2, are constructed with a closed panel timber system; the walls and roof have a U-value of 0.09 W/(m2K). The floor has a U-value of 0.12 W(m2K). Triple-glazed windows are used throughout. The certified house is fitted with a highly efficient MHRV unit which has a heat recovery rate between 92 and 99 per cent, and duct runs are short due to the house design. A solar thermal system provides domestic hot water.

Architect Gokay Deveci of Robert Gordon University sees PassivHaus as much more than an energy standard. ‘It is a quality assurance standard that closes the gap between theoretical performance and reality,’ he says. The houses have now been occupied for six months and energy data monitoring is underway.

The site of Underhill House is prominently located at the top of a hill in the Cotswolds Area of Outstanding Natural Beauty. Invisible from the surrounding countryside, the concrete house is built into the hillside under an existing barn which the planners determined should be retained and which will serve in future as a home office. Planning permission was granted under paragraph 11 of PPS7.

Two sides of the L-shaped underground house face south and are entirely glazed with high-performance triple glazing to maximise solar gain and minimise heat loss. Much of the concrete structure is left exposed internally to exploit its thermal mass; it is insulated and waterproofed externally. A large photovoltaic array will generate most of the home’s electricity.

The house has no boiler, only a whole-house mechanical ventilation system with heat recovery which has a small back-up heater connected to the hot water store. For hot water, one side of the barn roof uses a glass slate system with solar hot water collectors beneath them. It looks like a traditional slate roof, without the ‘bolt-on’ appearance of standard solar collectors, which together with the back-boiler from a small wood-burning stove will connect into a large thermal store.

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